Many people who sent their cameras in to Nikon for this issue reported that Nikon changed a whole assembly in the camera - which would indicate some manufacuring defect, if not a design defect.

However reports of this issue seem to have died right down over the past couple of months or so - which suggests that Nikon have solved the issue. Now the number of new "dirty/oily" sensor reports are down to about the same level they are with any other new Nikon DSLR.

Where-as undoubtedly people have/do have problems with oil on the sensor the occurrence of oil on the sensor is not a design defect per se but rather a QC issue where excess oil (or grease) was added to the mechanism. This would seem to be the obvious cause as frequent cleanings do seem to appear to reduce the issue.

The D600 is not the only camera from Nikon or Canon that has suffered from the issue as has been pointed out in other posts.

My December purchased D600 doesn't have any oil issues - and there are reports of other users not having the issue - so I would surmise that Nikon identified the issue early on when the reports came through and did something about it.

My regrets that others have had this issue as it undermines the otherwise great IQ this camera is capable of.